Semiconductor memory devices are widely used in the computer and electronics industries to store digital information. A typical semiconductor memory device has a large number of memory elements, known as memory cells, that are each capable of storing a single digital bit or data bit. Some memory cells store critical process data in numerical controls such as diagnostic and measurement systems and industrial and automotive electronics.
Static random access memory (SRAM) is well known in the art and consists of a bistable transistor flip-flop or latching circuit. The data retained in the SRAM cell is volatile, in that any interruption of the power supply voltage source causes loss of the data. An alternate to the volatile SRAM is non-volatile RAM (nvRAM). Non-volatile RAM consists of a floating gate transistor which has a charge placed on a floating gate to modify the voltage threshold of the floating gate transistor that indicates the state of the data retained in the nvRAM. Stored data is not lost upon interruption of power.
Non-volatile static random access memory (nvSRAM) is the product of non-volatile data retention as well as fast SRAM operation. An nvSRAM has two different modes of operation: SRAM mode and non-volatile mode. An nvSRAM takes advantages of fast data read/write operations (a benefit of the SRAM portion) and it is a solution where reliable data retention has to be guaranteed under extreme conditions (a benefit of the nvRAM portion). Although there are benefits of nvSRAM, there are also deficiencies.
One typical nvSRAM cell consists of a six transistor SRAM cell, commonly referred to as a 6T SRAM cell, and a six transistor electrically erasable programmable read-only memory, commonly referred to as a six transistor EEPROM or 6T EEPROM. In the 6T EEPROM, two of the six transistors are non-volatile transistors used for SRAM data store operations. The key operation of this product is normal SRAM operation, including bulk erase of non-volatile memory for data reset, programming non-volatile memory to store data from SRAM and data recall process from non-volatile memory to SRAM. However, as the technology aggressively scales down metal oxide semiconductor field effect transistors, (MOSFETs), the leakage currents of MOSFETs are increasing, which causes a high stand-by current as well as the degradation of data retention of the EEPROM. This also limits a scaling down of chip integrity of the current nvSRAM products. There are always needs for improvements in memory architecture.